Epoxy resins have good mechanical properties, great resistance to heat and chemicals, and high bond strength, so they are widely used as thermosetting resins is laminated sheets, adhesives or paints. Organic solvents have conventionally been used as a dispersion medium for forming epoxy resins and other synthetic resins into a thin film, but to meet the social demand for controlling pollution and saving resources, active efforts are being made to develop water-reducible resins that can replace the current organic solvent-dispersible resins.
Two techeniques are known for dispersing synthetic resins in water: one is (1) the forced emulsification method wherein a surfactant is used to cause the resin to be emulsified in a medium mainly composed of water, and the other is (2) the hydrophilic group grafting method wherein a polymerizable hydrophilic vinyl compound is grafted onto the resin. The first method is commonly employed to disperse epoxy resin in water, but this method has the following defects: (a) the resulting aqueous dispersion is not stable and the epoxy resin easily separate from water; (b) the epoxy resin is not dispersed uniformly (i.e. the size of the dispersed particles is not uniform), so the subsequent curing of the resin is not performed uniformly to produce a resin product having good properties; and (c) because of the use of a large amount of surfactant, the resulting resin film has poor water resistance and bonding strength and at the same time, the surfactant is released from the film into water.
The second method is a rather recent technique proposed in Japanese Patent Public Disclosure No. 1228/78, see U.S. Pat. No. 4,212,781, and it has the following defects: (a) since the method involves a reaction in an organic solvent, much organic solvent is necessary and the residual monomer is difficult to remove; and (b) the grafting efficiency of the method is low and the ungrafted portion of the resulting film easily separates from the grafted portion to thereby impair the physical properties of the film. Therefore, it has been desired in the art to develop a chemically stable, water-reducible resin composition that has great resistance to water and solvents as well as good physical properties. According to the present invention, a water-reducible epoxy resin composition is provided by irradiating a mixture of an epoxy resin and a radical-polymerizable vinyl compound with electron beams. There are two conventional methods whose concept is similar to that of the present invention: (1) a mixture of an epoxy resin and a radical-polymerizable monomer having a carboxyl group is irradiated with electron beams to make the epoxy resin water-reducible; (2) a mixture of an epoxy resin and a radical-polymerizable monomer having a nondissociative hydrophilic group such as a hydroxyl group or ether bonded group is irradiated with electron beams to thereby make the epoxy resin water-reducible. The epoxy resin to which the carboxyl-having monomer is grafted is made water-reducible by adding an alkali such as amine to dissociate the carboxyl group, but since the product contains not only the epoxy group but also the acid and amine that cause the reaction of opening the epoxy ring, the aqueous dispersion of the product cannot be stored for an extended period without changing its properties, and at the same time, a film of resin does not reflect satisfactorily the desired mechanical properties, such as high impact resistance, of the epoxy resin. The epoxy resin to which a nondissociative monomer having a hydroxyl group or ether bonded group is grafted is made water-reducible by adding water to the resin after it is dissolved or swollen in a suitable organic solvent. But to prevent the agglomeration of the dispersed resin particles, the specific resin formulation requires more rigorous limitations on the formulation and amount of the organic solvent and the proportion of water in the dispersion than those required for the aqueous dispersion of the resin to which a carboxyl group is grafted.